Presynaptic receptors are thought to be important in the regulation of transmitter release from a variety of neurons. The alpha2 adrenergic receptor is located upon noradrenergic, serotonergic, dopaminergic and cholinergic neurons and, when stimulated, inhibits neurotransmitter release. The proposed project is based upon the following hypotheses: 1) presynaptic alpha2 adrenergic receptors on noradrenergic neurons in the central nervous system are important sites for the pharmacological actions of psychotropic drugs, especially those psychomotor stimulants such as cocaine and the amphetamines which are known to release catecholamines from neurons and block neurotransmitter reuptake; 2) long-term administration of such drugs causes changes in presynaptic receptor number, affinity and/or function which differ from those produced by short-term drug administration, and those changes in alpha2 adrenergic receptor function after long-term psychomotor stimulant drug administration reflect those which occur in drug- dependent individuals; 3) psychomotor stimulants which produce dependence modify alpha2 adrenergic receptor function at specific cellular sites (e.g. alpha adrenergic receptors located upon nerve endings vs those located on cell bodies), upon specific types of neurons (e.g. noradenergic vs cholinergic neurons) and at specific sites in the brain (e.g. hippocampus vs brainstem). Receptor-ligand binding studies will be performed in order to determine whether the number and/or affinities of various adrenergic receptors change after either short- or long-term psychomotor stimulant drug administration. Functional changes in presynaptic alpha2 adrenergic receptors located upon noradrenergic neurons will be determined in three different preparations: 1) superfused, electrically stimulated rat brain slices 2) the isolated rat vas deferens preparation and 3) isolated rat left atrial strips. The release of neurotransmitter during electrical stimulation of neurons in these preparations will be determined by measuring one or more of the following parameters: a) the release of 3H-norepinephrine after incubating the tissues with 3H-norepinephrine or its precursor, 3H- dihydroxyphenylalanine, b) the efflux of endogenous neurotransmitter by a combination of high pressure liquid chromatography and electrochemical detection, c) the release of dopamine beta-hydroxylase, an enzyme which is released with norepinephrine but which is not taken back up into the neuron or d) changes in the responses of the isolated muscle preparations.